Page:Popular Science Monthly Volume 20.djvu/159

Rh the temperature and density of the surrounding waters may expand or contract that period considerably.

All authorities upon the early stages of the European variety concur in the statement that the young oyster or "spat" is formed by the fertilization of the eggs of the female while within the shell of that animal, and that the "spat" is held between the gills, and thus protected by the parent until the shell has formed. Many authorities are also of the opinion that the parents are hermaphrodites, and it is here that the first difference between the European and American varieties is noticed. Professor W. K. Brooks, in his recent paper upon the embryology of the American oyster, states that, though he has during the past two years examined a very large number of oysters, he has never found any evidence of a union of the two sexes in the same animal; and my own more limited observations have likewise failed to discover any such indication.

The second and most material difference is in the manner of impregnating the eggs of the female. According to the best authorities the fertilization of the eggs of the European variety is accomplished by the passage of the male fluid into the water, and thence between the valves and gills of the female, where the eggs, having been expelled from the ovaries, are lying ready for fertilization. The young resulting from the union of the ova and spermatozoa are held and protected within the gills of the female until the shells have formed and until they are quite well advanced in development, having at the time of their expulsion locomotive powers of their own, which enable them to swim about and seek an appropriate place for attachment.

The American variety, according to the observations of Dr. Brooks, differs in this, that the young oyster is not found within the gills of either parent, nor does the fertilization take place within the shell, but the contents of the generative organs of both sexes are expelled into the water, there to stand the chance of coming into contact. My own observations support this theory, inasmuch as relates to the presence of young within the gills, I having examined those parts of a large number of oysters very carefully, and with an utter failure to discover an embryo oyster, even during the height of the spawning.

It is evident that a large measure of protection is afforded the young of the European variety by the inclosing shells of the parent, and that this protection is given during the most precarious stages of their existence, while the ova and spermatozoa of the American oyster are not only left to a happy chance for their successful union, but the